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RE: Lateral Stability of a Box Beam ?

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Mr T. Boene,

Bills request for information for design of a box beam was to replace two
independent beams, with a single box beam, because if the effective length
is the full span, the two independent beams get too large. The choice of the
box beam was indicated to be to increase torsional resistance. There was no
mention of providing a simple plywood diaphragm to brace the two beams, or
any other form of bracing. Turning two beams into a box beam potentially
wider than deep, instead of providing bracing is not very efficient.

And yes the Australian timber structures code (AS1720) provides for what it
calls spaced columns: these can have either packer plates or batten plates
joining the sticks. It also covers built up sections: I-beams, boxes. The
steel structures code (AS4100) covers laced and battened columns.

Whether the two beams are considered as an assembly forming a horizontal
truss, or a battened column. Bracing the two beams would seem to be more
efficient than adding the extra weight of plywood covers top and bottom for
the full length, to lightly loaded beams: not supporting much more than
their own self weight.

Whilst Paul may be correct about the two beams displacing in unison:
implying a half sine wave buckle for the full span. The battened column
analogy indicates that such will be at a higher axial load, than for the
beams acting individually.

Galambos discusses lean on effects for columns and beams: and beams don't
appear to be a major concern (pg 471?). More over then goes on to design
bracing, which is similar to the bridging we use in Australia for coldformed

Which is where I think the issue is. Most of the photos I have seen of
bridging of purlins in the US, show bracing which appears little better than
sag rods. So my alternative proposal of providing bridging to the beams
wasn't a good one. Divided by a common language.

And you are right we are not disputing the benefits of box beams. Rather
whether a box beam is more efficient or suitable for the situation than
bracing. If bracing is the alternative, then suitable bracing then becomes
an issue.

Bill no doubt will accuse me of over complicating things again, for what is
probably a simple design problem.

Conrad Harrison
B.Tech (mfg & mech), MIIE, gradTIEAust
South Australia

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